Controlling pressure in a process chamber by variying pump speed and a regulator valve, and by injecting inert gas

ABSTRACT

A gas pumping system of the invention enables gas to be pumped from a process chamber ( 1 ) in order to regulate its pressure as a function of process steps. The system comprises a primary pump ( 2 ), a secondary pump ( 8 ), an inert gas injection device ( 6 ), and a regulator valve ( 4 ). The primary and/or secondary pump ( 2  and/or  8 ) is controlled in speed in order to regulate variations of greater amplitude. The regulator valve ( 4 ) is controlled in opening in order to regulate variations that are small amplitude and fast. The inert gas injection ( 6 ) is controlled in flow rate in order to regulate medium amplitude variations. This provides great reaction stability and a wide range of possible variation for the conditions in the process chamber ( 1 ).

The invention described in this document relates to controlling the pressure of gas in a process chamber used in particular in the semiconductor industry.

Methods of fabricating semiconductors and microelectronic mechanical systems (MEMS) generally comprise successive steps which take place in a process chamber under an atmosphere at low pressure. Each step is characterized by a gas pressure that needs to be regulated, e.g. in order to maintain a plasma or a particle bombardment acting on a semiconductor substrate.

Certain steps include simultaneous injection of treatment gas into the process chamber.

Most of the steps of the process take place in the presence of a suitable vacuum, generated and maintained by a vacuum line that includes vacuum pumps connected to the process chamber.

In traditional manner, pressure in process chambers has been controlled by operating a regulator valve placed directly at the outlet from the chamber prior to the secondary pump and the primary pump. A problem then arises with a risk of the regulator valve becoming dirty, and a risk of pollution being scattered back from the regulator valve into the process chamber.

A solution to this problem, consisting in simultaneously controlling the speeds of the primary and secondary pumps, constitutes the subject matter of U.S. Pat. No. 6,419,455. Difficulties arise when the primary pump is remote, being separated from the secondary pump by a pipe that is relatively long. The response time of the regulation system is then too long.

Also known, from document WO 99/04325, are a large number of solutions of varying complexity, making use:

-   -   sometimes of controlling the speed of the primary pump while         injecting an inert gas upstream from the primary pump and         downstream from an outlet valve from the secondary pump; and     -   sometimes injecting inert gas upstream from a control valve         connected to the inlet of the primary pump whose speed is not         controlled.

However, that document does not describe a solution in which a regulator valve is engaged between a primary pump and a secondary pump, the speed of the primary pump being controlled, and an inert gas being injected upstream from the regulator valve. Nor does that document describe a solution in which the speed of the secondary pump is controlled.

A difficulty in controlling the atmosphere in process chambers for fabricating microtechnical and microelectronic components lies in the wide variety of machining steps making use of plasmas or other gaseous elements, and the wide variety of physical conditions for the atmosphere that is present in the process chamber. The control device must be capable of following these variations which are of relatively large amplitude. It is also necessary for the control device to follow variations quickly so as to provide machining steps that are performed correctly, complying from the beginning to the end of each step with the appropriate conditions for the machining. Otherwise, a machining step cannot begin until after the atmosphere in the process chamber has stabilized, thereby reducing the fabrication throughput rate and increasing the cost of production.

Prior art documents do not give satisfactory teaching on achieving reaction speeds and adjustment ranges that are appropriate for all of the steps that are necessary.

Thus, the invention seeks simultaneously to avoid the drawbacks of prior art systems, in particular by making it possible significantly to reduce the regulation response time of the system and to reduce risks of instability, while also increasing adjustment range, and simultaneously avoiding any risk of back-scattering pollution which might result from a control valve being present at the outlet from the process chamber.

The invention thus seeks to replace the regulator valve at the outlet from the chamber by satisfactory means which achieve appropriate reaction speed and amplitude even in the presence of a long pipe between the primary and secondary pumps.

The invention also seeks to guarantee satisfactory regulation stability, avoiding the appearance of the instabilities that are frequently encountered in regulated systems when it is desired to increase the amplitude and the speed of reaction.

The essential idea of the invention is to perform regulation by three complementary means whose reaction speeds complement one another:

-   -   controlling the speed of the primary and/or secondary pump, thus         making it possible to respond to very long-term trends;     -   injecting inert gas under flow rate control, at a point located         upstream from a regulator valve itself, and upstream from the         primary pump, thereby responding to medium-term trends; and     -   controlling the opening of the regulator valve, thus providing a         reaction that is very fast when placed under appropriate         operating conditions by injecting gas and regulating the speed         of the primary pump.

To achieve these objects amongst others, the invention provides a system for pumping gas from a process chamber, the system comprising a primary pump, a secondary pump, an inert gas injection device, and a regulator valve, in which:

-   -   the primary and/or secondary pump is controlled in speed;     -   the regulator valve is controlled in opening and is connected to         the inlet of the primary pump; and     -   the inert gas injection is controlled in flow rate, and is         provided upstream from the regulator valve.

The combination of these elements gives great flexibility in adjustment, making it possible to achieve a very fast reaction speed and a broad range of possible adjustments.

The invention is preferably applied to a speed-controlled secondary pump of the turbo, drag, or turbo/drag pump type.

In a preferred embodiment, the regulator valve is controlled so as to compensate for fast variations of small amplitude in conditions of pressure and gas injection flow rate in the process chamber, inert gas injection is controlled to compensate for larger amplitude variations in conditions of pressure and gas injection flow rate in the process chamber, and primary and/or secondary pump speed is controlled so as to compensate for longer-term trends and greater variations in the amplitude of conditions of pressure and gas injection flow rate in the process chamber.

In this case, the speed(s) of the primary and/or secondary pump and the injection of gas are preferably determined so as to place the regulator-valve in an appropriate intermediate opening position, which is a function of the selected valve. The person skilled in the art can adapt the mean position of the regulator valve as a function of response curves provided by the valve manufacturer. The regulator valve is thus placed in the mean position that gives the valve its most appropriate reaction sensitivity over the fluid it controls as a function of its position.

Likewise, the person skilled in the art will preferably chose to adapt the pump speeds or the injection of gas as a function of the sensitivity desired for regulation purposes, preferring to adapt gas injection in order to achieve high regulation sensitivity, or to adapt gas injection and pump speed in order to reduce regulation sensitivity.

In a practical embodiment, the pumping system of the invention may be such that:

-   -   the primary and/or secondary pump is connected to control means         for controlling the primary and/or secondary pump at adjustable         speed;     -   the regulator valve is associated with control means for         controlling the regulator valve;     -   the inert gas injection injects inert gas from an inert gas         source via an injection pipe provided with an injection valve         and control means for controlling the injection valve; and     -   central control means such as a microprocessor or a         microcontroller control the respective control means for the         primary and/or secondary pump, for the regulator valve, and for         the injection valve.

Preferably, the central control means generate the signals which control the respective control means as a function of a reference signal received from reference means, as a function of information concerning the opening positions of the regulator valve and of the injection valve, and as a function of measurements of the pressure in the process chamber as issued by a pressure sensor.

The invention may advantageously be applied to making an installation for fabricating semiconductors or microelectronic mechanical systems (MEMS).

In another aspect, the invention provides a method of fabricating semiconductor or microelectronic mechanical systems (MEMS) in which the atmosphere inside a process chamber is pumped by means of a pumping system as defined above.

Preferably, the opening of the regulator valve is controlled in such a manner as to compensate for rapid variations of small amplitude in the conditions of pressure and gas injection flow rate in the process chamber, the inert gas injection is controlled in flow rate so as to compensate for larger amplitude variations in the conditions of pressure and gas injection flow rate in the process chamber, and the speed of the primary and/or secondary pump is controlled in such a manner as to compensate for longer-term trends and for variations of large amplitude in the conditions of pressure and/or gas injection flow rate into the process chamber.

Other objects, characteristics, and advantages of the present invention appear from the following description of particular embodiments described with reference to the accompanying figures, in which:

FIG. 1 a is a timing diagram showing an example of variation in the reference pressure for a process chamber, together with the real variation in pressure;

FIG. 1 b shows an example of simultaneously varying flow rates of gas introduced into the process chamber during treatment;

FIG. 1 c then shows a curve for variation in the opening of a valve in order to follow small-amplitude pressure and flow rate variations in the process chamber;

FIG. 1 d is a curve showing how the variation in gas injection that needs to be implemented in order to follow long-term trends or variations of greater amplitude;

FIG. 1 e is a curve showing variation in the speed of the primary pump;

FIG. 2 is a block diagram of apparatus for controlling pressure in a process chamber in an embodiment of the present invention; and

FIG. 3 shows regulation sensitivity curves as a function of valve opening for different pump speed and gas flow rate conditions.

In the embodiment shown in FIG. 2, in a vacuum line for controlling the vacuum in a process chamber 1, there are provided a primary pump 2 connected to means 3 for controlling the primary pump 2 at adjustable speed. A regulator valve 4 is provided at the inlet to the primary pump 2, in association with means 5 for controlling the regulator valve 4. Inert gas injection 6 under flow rate control is provided by injecting the inert gas upstream from the regulator valve 4 from an inert gas source 7 via an injection pipe 7 a provided with an injection valve 7 b and with means 7c for controlling the injection valve 7 b. The primary pump 2 is connected to a secondary pump 8 which is itself connected to the process chamber 1 possibly via an isolating valve 9. The secondary pump 8 is itself connected to means 11 for controlling the secondary pump 8 at variable speed.

The means 3 and 11 respectively for controlling the primary pump 2 and the secondary pump 8 define the speed of rotation of the primary pump 2 or of the secondary pump 8,.and they stabilize on a determined value as explained below. Similarly, the means 7c for controlling inert gas injection 6 define the rate at which inert gas is injected upstream from the regulator valve 4 at a rate that is appropriate, as defined below. Finally, the means for controlling the regulator valve define an appropriate opening position for the regulator valve 4.

For a given speed of the primary pump 2 and/or the secondary pump 8, the inert gas injection means 6 are programmed to inject a quantity of inert gas that enables an appropriate mean gas pressure to be achieved downstream from the secondary pump 8, thereby determining a mean pressure in the process chamber 1 that corresponds to the mean pressure required by the current step of the method, for a mean position of the regulator valve which is intermediate being in a zone of sensitivity that is appropriate for the adjustment.

The adjustment sensitivity of the regulator valve is illustrated by curve A in FIG. 3 which shows the slope of pressure variation in the process chamber 1 as a function of the opening position of the regulator valve 4 for given downstream pressure. The regulator valve 4 is preferably placed in a position of intermediate opening in the range O running from 40% to 60%, corresponding to a satisfactory variation slope for the combined transfer function of the valve and the secondary pump 8.

During the process steps, the mean pressure in the process chamber, and the mean flow rate of gas injected into the process chamber can vary. To track said variation in mean pressure and flow rate, it would be necessary to cause the rate at which inert gas is injected upstream from the regulator valve 4 to be caused to vary considerably. In order to avoid reaching flow rates that are too high or zero, the means 3 and/or 11 for controlling the primary pump 2 and/or the secondary pump 8 at adjustable speed adapt the speed of the primary and/or secondary pumps 2 and/or 8 so as to achieve an appropriate mean pressure at the inlet to the primary pump 2 for a mean inert gas flow rate 6 and for a mean opening of the regulator valve 4.

In addition, because of the satisfactory adjustment and above all because of the wide range of adjustment possibilities, it is possible to perform regulation quickly in a very broad range of variation in the conditions present in the process chamber 1.

The means for regulating inert gas injection and the means for controlling the regulator valve 4 can be combined with regulating the speed of the primary pump 2 alone, or with regulating the speed of the secondary pump 8 alone, or with regulating the speeds of both the primary pump 2 and the secondary pump 8. This two-pump possibility makes it possible to further increase the range of variation that is possible in the conditions that exist inside the process chamber, and make it possible to further increase the reaction speed of control.

FIGS. 1 a to 1 e show an example of how operating conditions in a system for controlling pressure in a process chamber can vary as a function of time in an embodiment of the invention.

FIG. 1 a shows firstly the reference pressure for the inside of the process chamber during a process sequence that is taken by way of example. It should be observed that the reference pressure begins at a relatively high value, of the order of 90 millitorr (mTorr), after which it falls very low, subsequently to rise to about 20 mTorr for a period until time mark 50.00, after which it falls very low again for a period subsequently rising to about 20 mTorr. Thereafter the reference pressure rises to about 40 mTorr round about time mark 100.00, falling subsequently to a low value and subsequently rising to about 60 mTorr prior to falling very low and subsequently rising to about 90 mTorr.

FIG. 1 a also shows the pressure actually achieved in the process chamber, and it can be seen that pressure drops quickly for negative steps in the reference pressure, but increases more slowly for positive steps in the reference pressure.

FIG. 1 b shows simultaneous variations in the flow rate of treatment gas introduced into the process chamber, said flow rate sometimes going against the variations in the reference pressure. The pumping system must then evacuate the process gas flow rate in order to track the reference pressure.

FIG. 1 c shows variations in the opening of the regulator valve 4.

FIG. 1 d shows variations in the rate at which inert gas is injected upstream from the regulator valve 4, while FIG. 1 e shows variations in the speed of the secondary pump.

Particular consideration can be given to the event which takes place around time mark 100.00. There is then simultaneously a decrease in the treatment gas injection flow rate into the process chamber (FIG. 1 b) and an increase in the reference pressure for the inside of the process chamber (FIG. 1 a). In order to achieve the desired pressure inside the process chamber, the regulator valve 4 is initially closed quickly, as indicated by negative step O1 in FIG. 1 c. Simultaneously, or slightly later as shown in the figures, the inert gas injection flow rate 6 is increased quickly upstream from the regulator valve 4, as represented by positive step F1. Given that the variations in injection flow rate and valve opening do not suffice to track the positive step in the reference pressure when in the presence of a decrease in the treatment gas flow rate into the process chamber, a variation in pump speed is also applied, as represented by negative step V1. The system responds correctly because of the three variations: in valve opening O1, in injection flow rate F1, and in pump speed V1. This enables the invention to follow variations of great amplitude.

A second advantage of the invention is explained below with reference to FIG. 3 showing how selecting which elements to regulate makes it possible to act on the overall regulation sensitivity of the system.

In this figure, curve A is a transfer or pressure variation curve for the process chamber 1 for various opening positions of the regulator valve 4 under given and constant conditions of speed in the pumps 2 and 8 and of inert gas injection flow rate 6 upstream from the regulator valve 4. From curve A, it will be understood that it is advantageous to place the regulator valve 4 in an intermediate opening position, advantageously in the range 40% to 60% of fully open, so as to conserve sensitivity that is sufficient but not too great.

Curve B shows that the transfer curve A is shifted towards higher pressures in the presence of variations in the inert gas injection flow rate 6 upstream from the regulator valve 4. Curve B remains relatively parallel to curve A. Similarly, curve C shows the transfer curve A being shifted towards high pressures for additional variation in the inert gas injection flow rate 6 upstream from the regulator valve 4.

Curve D shows how the transfer curve becomes deformed in the event of simultaneously varying the inert gas injection flow rate upstream from the regulator valve 4 and the speed of the primary pump 2. Finally, curve E shows how the transfer curve is deformed in the event of the speed of the primary pump 2 and the inert gas injection flow rate 6 upstream from the regulator valve 4 being caused to vary for a greater injection flow rate.

These curves can be interpreted as follows: if the valve is placed at a mean opening of 50%, a progressive increase in the inert gas injection flow rate 6 upstream from the regulator valve 4 causes the transfer curve to shift from a point A1 towards a point B1 and then towards a point C1 on the respective curves A, B, and C. It can be seen that at point B1, the slope of the curve B is steeper than it is at point A1 on curve A, and that it is even steeper at point C1 on curve C. This means that by increasing the inert gas injection flow rate 6 upstream from the regulator valve 4, regulation sensitivity is progressively increased. In some cases, this can lead to instabilities, due to excess reaction gain.

Still for an opening at 50% for the valve, consideration is given to point D1 on curve D and point E1 on curve E, to which it can be seen that curves D and E present much shallower slope for the same pressure conditions in the process chamber. Thus, by choosing also to vary the speed of the primary pump, in order to move onto curves D and E, it is possible to reduce regulation sensitivity and thus to occupy a zone of improved stability.

Advantage can thus be taken of the means of the invention to adapt pump speed(s) or inert gas injection in preferred manner as a function of the sensitivity desired for regulation purposes. A constant pump speed enables greater regulation sensitivity to be achieved merely by adapting gas injection while also adapting the opening of the regulator valve 4. In contrast, changing pump speed while simultaneously adapting inert gas injection 6 upstream from the regulator valve 4 makes it possible to reduce regulation sensitivity so as to improve regulation stability.

With reference again to FIG. 2, there can be seen a diagram for a practical organization of the means for controlling the various members of the vacuum line of the invention.

The control means 5 and 7 c for respective valves 4 and 7 b may be electromagnetic interfaces which move the shutter means of the respective valves mechanically as a function of a reference value received from central control means 10 such as a microprocessor or a microcontroller to which they are connected. The same microprocessor or microcontroller 10 also controls the supply of electricity to the primary pump 2 and/or to the secondary pump 8 via the control means 3 and/or the control means 11 corresponding thereto and thus constituting power supply control means connected to the central control means 10.

The central control means 10 such as the microprocessor or microcontroller may optionally receive information concerning the opening position of the regulator valve 4 and the injection valve 7 b, and concerning the speed(s) of the primary and/or secondary pumps 2 and/or 8, via sensors that are not shown in the figure. A sensor may also be provided for picking up the injection flow rate into the pipe 7 a. The microcontroller also receives signals representing pressure measurements made in the process chamber 1 and issued by a pressure sensor 12, and it also receives the pressure reference signal as generated by reference means 13.

Alternatively, the central control means 10 could issue control signals in an open loop configuration without receiving measurement signals.

The present invention is not limited to the embodiments described above, but it includes the variants and generalizations that are within the competence of the person skilled in the art. 

1. A system for pumping gas from a process chamber (1), the system comprising a primary pump (2), a secondary pump (8), an inert gas injection device (6), and a regulator valve (4), in which: the primary and/or secondary pump (2 and/or 8) is controlled in speed; the regulator valve (4) is controlled in opening and is connected to the inlet of the primary pump (2); and the inert gas injection (6) is controlled in flow rate, and is provided upstream from the regulator valve (4).
 2. A pumping system according to claim 1, in which the regulator valve (4) is controlled so as to compensate for fast variations of small amplitude in conditions of pressure and gas injection flow rate in the process chamber (1), inert gas injection (6) is controlled to compensate for larger amplitude variations in conditions of pressure and gas injection flow rate in the process chamber (1), and primary and/or secondary pump (2 and/or 8) speed is controlled so as to compensate for longer-term trends and greater variations in the amplitude of conditions of pressure and gas injection flow rate in the process chamber (1).
 3. A pumping system according to claim 2, in which the speed(s) of the primary and/or secondary pump (2 and/or 8) and the rate of inert gas injection (6) are determined so as to place the regulator valve (4) in a mean position of appropriate intermediate opening in which the regulator valve presents appropriate reaction sensitivity.
 4. A pumping system according to claim 1, in which: the primary and/or secondary pump (2 and/or 8) is connected to control means (3 or 11) for controlling the primary and/or secondary pump (2 and/or 8) at adjustable speed; the regulator valve (4) is associated with control means (5) for controlling the regulator valve (4); the inert gas injection (6) injects inert gas from an inert gas source (7) via an injection pipe (7 a) provided with an injection valve (7 b) and control means (7 c) for controlling the injection valve (7 b); and central control means (10) such as a microprocessor or a microcontroller control the respective control means (3, 11, 5, 7 c) for the primary and/or secondary pump (2 and/or 8), for the regulator valve (4), and for the injection valve (7 b).
 5. A pumping system according to claim 4, in which the central control means (10) generate the signals which control the respective control means (3, 11, 5, 7 c) as a function of a reference signal received from reference means (13), as a function of information concerning the opening positions of the regulator valve (4) and of the injection valve (7 b), and as a function of measurements of the pressure in the process chamber (1) as issued by a pressure sensor (12).
 6. A system for fabricating semiconductor or microelectronic mechanical systems (MEMS), the installation including at least one process chamber (1) and a pumping system according to claim
 1. 7. A method of fabricating semiconductor or microelectronic mechanical systems (MEMS), in which the atmosphere inside a process chamber (1) is pumped using a pumping system according to claim
 1. 8. A method according to claim 7, characterized in that the opening of the regulator valve (4) is controlled in such a manner as to compensate for rapid variations of small amplitude in the conditions of pressure and gas injection flow rate in the process chamber (1), the inert gas injection (6) is controlled in flow rate so as to compensate for larger amplitude variations in the conditions of pressure and gas injection flow rate in the process chamber (1), and the speed of the primary and/or secondary pump (2 and/or 8) is controlled in such a manner as to compensate for longer-term trends and for variations of large amplitude in the conditions of pressure and/or gas injection flow rate into the process chamber (1). 